Low fuel pressure, PA24-250

[Don Ostergard]

At the risk of drifting slightly off topic, I'd like to explain a very simple and valuable mod to the aux fuel pump circuit that I performed on my 1959 - 250 about 30 years ago (I've owned it for 37). I had always been unhappy with the two pumps being controlled by the same switch. In case of switch failure or wire breakage, or in case of short circuit in either aux pump or anywhere else between and including the switch, the use of both aux pumps is lost. Thus losing a very desirable degree of redundancy. So I rewired the two pumps to be controlled individually, using a split type Cessna master switch which fits nicely into the space formerly occupied by the original toggle switch. Now, one can check the operation of each pump individually prior to startup, with absolute confidence in what each pump is doing. Saves a bit of time during Annual, too, as it is so easy to test the output of each pump - no longer any need to disconnect any wires.

This modification appears in "Tips Special" as "Electrical Fuel Pump Modification" in the "Fuel System" section. This submission also includes a wiring diagram to help visusalize the project.

Don Ostergard,ICS 3263
Drumheller AB

[Tom Deml]

Well, still not totally corrected. New electrics are installed and are working properly. And we have solved some of the symptoms. We have also isolated the airframe fuel source(s) and ran a totally separate source for fuel to verify we don't have an airframe side airleak. However the engine pump, with clear hose installed so you can see, does not produce enough pressure to fill the line after the electrics are turned off at a medium to higher power setting. 6" to 8" of air results in the line with a "trickle" of fuel on the outlet side of the engine pump. It is a new pump from the overhaul and we have tried another new pump. Same problem. I know I read an article somewhere, not sure if it was in the Comanche database or not, about the proper plunger length that drives the fuel pump. The parts manual shows the same part number for the plunger and refers to it as "long". Does anyone know the specs on this plunger? Is it possible there are different lengths under the same part number?? The engine driven pump is not creating enough pressure and could it be the arm of the pump is not being driven the proper travel distance by the plunger? My overhaul shop has not given me a definitive......I've gotten the typical answer so far of "haven't seen this before".
Also note, this change happened after the overhaul. I noticed fuel pressure issues immediately after the overhaul one year ago. However, with the electrics turned on, things stabilized. However then the electrics were failing, unbeknownst to me, which complicated this issue. But now I think we are down to an issue that is a result of the overhaul and some sort mechanical change.

[Tom Deml]

Discovered the two new electric fuel pumps we ordered were defective. Their check valve systems were not working properly. This thru us for a loop as we chased gremlins once again assuming we had two new and working pumps. To test we disconnected fuel source from the airframe at the firewall. Isolated the electrics and plumbed in an air pressure gauge prior to the carb. Blocked off the engine driven pump. Air pressurized to 5 psi on the outlet side of the electrics, opened the plumbed inline valve and you could feel the air quickly release out the intake side of each new electric. Had two other pumps then ordered and bench tested before shipping. Did the same test on them once we received them. They held the pressure (not completely sealed but certainly as designed). Hooked up the fuel system, and MAGIC....working thru all power phases on the ground. Need to get the cowling back on and get things buttoned up and flight test.